Sounds like you should be trying a D cup. Is the lady at the lingerie shop impressed at how big you are getting after having started out as a B cup?
Seems like you are still growing, so I wonder how big you’ll end up.
The days of going braless are gone for good. You need to wear a bra now as much as any woman does, both for comfort and appearance. I can understand why you have moved up to underwire bras, as they give more support. They also give more uplift and shaping, which probably suits your breasts, as you have noted they are now quite feminine in appearance, as opposed to being moobs.
What kind of clothing, including bra, advice did your female friend who noticed you wearing a bra give you? Did you discuss your bra size? She have any reaction to you being a C cup, still growing and maybe already a D cup?